The present invention relates to temperature control systems for maintaining the temperature of an integrated circuit chip (IC-chip) near a constant set point temperature while the IC-chip is being tested. Also the present invention relates to subassemblies which comprise key portions of the above temperature control systems.
The IC-chip whos temp rature is being regulated typically is part of an integrated circuit module. In the IC-module, the IC-chip usually is mounted on a substrate and covered with a lid. Alternatively, an uncovered IC-chip can be mounted on the substrate. Any type of circuitry can be integrated into the IC-chip, such as digital logic circuitry, or memory circuitry, or analog circuitry. Further, the circuitry in the IC-chip can be comprised of any type of transistors, such as field effect transistors or bipolar transistors.
One reason for trying to keep the temperature of an IC-chip constant, while the IC-chip is tested, is that the speed with which the IC-chip operates may be temperature dependent. For example, an IC-chip which is comprised of complementary field effect transistors (CMOS transistors) typically operates faster as the temperature of the IC-chip is decreased.
A common practice in the IC-chip industry is to mass produce a particular type of IC-chip, and thereafter speed sort them and sell the faster operating IC-chips at a higher price. CMOS memory chips and CMOS microprocessor chips are processed in this fashion. However, in order to properly determine the speed of such IC-chips, the temperature of each IC-chip must be kept nearly constant while the speed test is performed.
Maintaining the IC-chip temperature near a constant set point is relatively easy if the instantaneous power dissipation of the IC-chip is constant, or varies in a small range, while the speed test is being performed. In that case, it is only necessary to couple th IC-chip through a fixed thermal resistance to a thermal mass which is at a fixed temperature. For example, if the maximum IC-chip power variation is only ten watts, and the thermal resistance between the IC-chip and the thermal mass is 0.2 degrees centigrade per watt, then the maximum variation in the IC-chip temperature will only be two degrees centigrade.
But, if the instantaneous power dissipation of the chip varies up and down in a wide range while the speed test is being performed, then maintaining the IC-chip temperature near a constant set point is very difficult. Each time the power dissipation in the IC-chip makes a big change, its temperature and its speed will also make a big change.
The instantaneous power dissipation of a present day microprocessor chip typically varies from zero to over one-hundred watts. Also, the trend in the IC-chip industry is to continually increase the total number of transistors on an IC-chip, and that increases the maximum power dissipation of the IC-chip. Further, in one type of test that is called “burn-in”, the power dissipation in the IC-chip is larger than normal because the voltage to the IC-chip is increased in order to accelerate the occurrence of failure.
In the prior art, one control system for maintaining the temperature of a high power IC-chip near a set point while the IC-chip is tested is disclosed in U.S. Pat. No. 5,821,505 (entitled “TEMPERATURE CONTROL SYSTEM FOR AN ELECTRONIC DEVICE WHICH ACHIEVES A QUICK RESPONSE BY INTERPOSING A HEATER BETWEEN THE DEVICE AND A HEAT SINK”). The '505 temperature control system includes a thin flat electric heater which has one surface that gets pressed against a corresponding surface on the IC-module, and has an opposite surface which is rigidly connected to a cooling jacket that carries a liquid coolant. The corresponding surface of the IC-module can be the lid which covers the IC-chip, or the IC-chip itself if there is no lid.
To cool the IC-chip at a maximum rate in the '505 temperature control system, the electric heater is turned off. Then heat quickly travels from the IC-chip through the electric heater to the cooling jacket. To reduce the rate at which heat travels from the IC-chip through the electric heater to the cooling jacket, the electric heater is turned on at a low level. To add heat to the IC-chip, the electric heater is turned on at a high level.
However, in the '505 temperature control system, a thermal resistance exists between the surfaces of the electric heater and the corresponding surface of the IC-module that get pressed together. This thermal resistance occurs due to microscopic mismatches between the two contacting surfaces.
The above thermal resistance times the power dissipation in the IC-chip equals a rise in temperature which occurs from the electric heater to the IC-chip when the electric heater is turned off to cool the IC-chip. This temperature rise limits the maximum power dissipation which can occur in the IC-chip without causing the IC-chip to overheat and destroy itself. Thus, the maximum power dissipation which can be tolerated without destroying the IC-chip is limited by the magnitude of the thermal resistance.
Further in the '505 temperature control system, the electrical heater inherently has a thermal mass. The larger that thermal mass is, the longer it takes to change the temperature of the electrical heater. Consequently, the thermal mass of the electrical heater limits the speed at which the temperature of the IC-chip can be regulated.
Accordingly, a primary object of the inventions which are disclosed herein is to provide a totally different structure for a temperature control systems, which completely avoids the above limitation of the '505 temperature control system.